1. Field of Invention
This invention relates to a composite filtration media for removing particulate matter entrained in a gas. More specifically, the invention relates to a composite filtration media useful in a vacuum cleaner.
2. Description of Related Art
Traditional vacuum cleaner bags possess a two-ply structure consisting of a wet-laid tissue fleece on the inside of the bag (up-stream of air flow) and filter paper on the outside of the bag (down-stream of air flow). The tissue fleece serves as a pre-filter removing only the largest dust particles. The filter paper typically has a porous structure that retains particles greater than about 10-20 μm.
It is known that the air flow and dust collection efficiency of these vacuum bags can be improved by replacing filter paper with fibrous fleeces such as meltblown (MB) ultra-fine webs, also referred to as “filtration grade MB fleece.” MB fleece can simultaneously serve as a pre-filter, coarse filter, and fine filter. Often, MB fleeces are used in conjunction with a mechanical support layer either upstream or downstream of the MB fleece. For example, an MB fleece can be paired with a spunbond (SB) layer that not only provides mechanical support for the MB fleece, but also abrasion resistance for the outside surface of the bag.
Typical MB fleeces can remove particles smaller than 5 μm. Although MB fleeces have a much lower porosity than traditional vacuum cleaner filter paper, they cannot hold dust as effectively unless they are treated. To enhance filtration efficiency, the MB fleece is often electrostatically charged.
Since MB fleece serves to filter both small and large particles, it has a propensity to clog up more quickly than traditional filter paper. Clogging of the filter media restricts air flow through the vacuum bag which leads to decreased suction performance of the vacuum cleaner. To mitigate this clogging effect, a coarse filter layer can be incorporated upstream of the MB fleece. The coarse filter layer, in effect, functions as a pre-filter for very large particles of dust which constitute a bulk of the dust typically encountered by vacuum cleaners. Since the coarse filter layer holds a significant proportion of the dust, the MB fleece layer can serve more effectively as a filter of fine particles with minimal clogging.
Such a coarse filter layer, which is also known as “high dust holding capacity” or “high capacity” paper, is typically disposed on the most upstream layer inside the bag, although certain vacuum bags also utilize lightweight tissue fleece, netting or other scrim on the most inside layer upstream of the coarse filter paper. Known materials for such coarse filter layers include wet-laid high capacity paper, dry-laid high capacity paper, high bulk meltblown nonwoven, modular spunblown nonwovens, and split film fibers.
Although several materials are known for use as coarse filter layers in vacuum cleaner bags, there remains a need in the art for filter materials with performance that is as good as, or better than, known materials for coarse filters, but are more economical, simpler to produce, and easier to incorporate into a filter bag. The present invention satisfies these needs among others.